Originally Posted by defjef
Did you mean that fiber glass cannot hold threads? I believe West believes that they can.
The problem is the pitch
of the threads, the pilot hole and the number of threads engaged. Why wouldn't GRP have no withdrawal strength? My own experience with tapping screws in thick GRP is that the head
can break off from screwing the down way more often than them spinning around and not holding.
I expect that West Systems might be referring to epoxy’s
ability to hold a thread, not an FRG laminate. A specific West citation might help clarify their position (link, direct quote).
The conventional purpose of the epoxy plug
, that Del & others describe in sandwich (cored) assemblies, is to provide a moisture barrier*, and crush resistance for through-bolted fastenings in compression
. These plugs may very well hold a screw thread, but I’ll stand by my advice that screws serve no structural purpose on a boat.
* By the same token, the raw edges of cut/drilled FRG laminates should be “sealed” with epoxy.
A thicker laminate of GRP will hold a screw better than will a thinner one; but neither will hold to any significant structural specification (in blind attachment).
Perhaps the breaking screw-head was a result of binding forces (high installation
or insertion torque required), but not due to the inherent pull-out resistance of an FRG laminate/screw combination. The fibrous nature of fibreglass may contribute to both ”binding” and poor pull-out resistance.